Hi, could you paste your full code in your sidebar.php? This code is missing some important portions.
You could use some free service as http://www.pastie.org/ that also allows PHP syntax highlighting. (then paste your resulting url here!)

However you could try and add this code:<?php if (function_exists('WPPP_show_popular_posts')) WPPP_show_popular_posts(); ?>
just after your </ul> in the end.
However, this depends on your theme.
Could you test if it is working for you?
Thank you! 🙂